The present invention relates to the recovery of carbon dioxide from gaseous streams containing it.
Conventionally, merchant liquid CO2 is produced from feed streams with high CO2 content ( greater than 95%) using distillation technology. Examples of such sources include ammonia and hydrogen plant off-gases, fermentation sources and naturally-occurring gases in CO2-rich wells. Typically, liquid CO2 is produced at a central plant and then transported to users that could be hundreds of miles away; thereby incurring high transportation costs. The lack of high quality sources and their distance from customers provides motivation to recover CO2 from low concentration sources, which are generally available closer to customer sites. Predominant examples of such sources are flue gases, which typically contain 3-30% CO2 depending upon the fuel and excess air used for combustion.
To produce merchant liquid CO2 from such sources, the CO2 concentration in the feed gas needs to be first upgraded significantly and then sent to a distillation unit. A variety of technologies, including membranes, adsorptive separation (e.g. pressure swing adsorption (xe2x80x9cPSAxe2x80x9d), vacuum pressure swing adsorption (xe2x80x9cVPSAxe2x80x9d) and temperature swing adsorption (xe2x80x9cTSAxe2x80x9d)), physical absorption and chemical absorption, can be used for upgrading the CO2 purity. The economics (capital and operating costs) of the overall scheme depends upon the purity of the feed, the product purity specifications and recovery obtained. For membranes, adsorptive separations and physical absorption, the cost to obtain a given high product purity is a strong function of the feed purity. On the other hand, chemical absorption provides a convenient means of directly obtaining high purity ( greater than 95%) CO2 vapor in a single step because the costs of this technology are relatively insensitive to the feed CO2 content. This vapor can be used as is for applications at the site of CO2 separation or further compressed for downstream recovery, as merchant liquid CO2, or for disposal/sequestration.
Chemical absorption can be performed through the use of alkanolamines as well as carbonate salts such as hot potassium carbonate. However, when using carbonate salts, it is necessary for the partial pressure of CO2 to be at least 15 psia to have any significant recovery. Since flue gases are typically available at atmospheric pressure, use of chemical absorption with carbonate salts would require compression of the feed gas. This is highly wasteful because of the significant energy expended in compressing the nitrogen. On the other hand, there exist alkanolamines that can provide adequate recovery levels of CO2 from lean sources at atmospheric pressure. Thus for recovery of high purity ( greater than 95%) CO2 vapor from sources such as flue gases, chemical absorption with alkanolamines would be the preferred choice. The pressure of CO2-rich vapor recovered from such an absorption process is generally around 15-30 psia. Compression of the gas will typically be needed for further use, processing or disposal.
Historically, alkanolamines have found widespread use for CO2 absorption in processes such as natural gas purification and hydrogen production. As the literature indicates (Kohl and Nielsen, xe2x80x9cGas Purificationxe2x80x9d, 5th Edition (1997), pp. 115-117, 123-125, 144-149), the feed gas is typically in excess of 200 psia and CO2-rich vapor is typically obtained at pressures of 15-30 psia. U.S. Pat. No. 5,853,680 discloses a process for the removal of carbon dioxide from high pressure ( greater than 425 psia) natural gas. There is no pumping of the CO2-rich alkanolamine liquid. By carrying out the regeneration step without significant depressurization, the disclosed process facilitates recovery of a CO2-rich vapor stream at pressures of 140 psia or higher.
However, there still exists a need for a more efficient process that can directly recover high pressure carbon dioxide from low pressure source streams.
One aspect of the present invention is a process for recovering carbon dioxide, comprising (A) providing a gaseous feed stream comprising carbon dioxide, wherein the pressure of said feed stream is up to 30 psia; (B) preferentially absorbing carbon dioxide from said feed stream into a liquid absorbent fluid comprising an organic amine absorbent to form a carbon dioxide enriched liquid absorbent stream; (C) in any sequence or simultaneously, pressurizing said carbon dioxide enriched liquid absorbent stream to a pressure sufficient to enable the stream to reach the top of the stripper in step (D) at a pressure of 35 psia or greater, and heating the carbon dioxide enriched liquid absorbent stream to obtain a heated carbon dioxide enriched liquid absorbent stream; and (D) stripping carbon dioxide from said carbon dioxide enriched liquid absorbent stream in a stripper operating at a pressure of 35 psia or greater and recovering from said stripper a gaseous carbon dioxide product stream having a pressure of 35 psia or greater. In some preferred embodiments, the pressure in the stripper, and the pressure of the gaseous carbon dioxide product stream, are up to 70 psia.
In other aspects of this process, the stripped liquid absorbent fluid from the stripper is recycled to step (B).